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<div class="titlepage"><div><div><h5 class="title">
<a name="fusion.algorithm.query.functions.count_if"></a><a class="link" href="count_if.html" title="count_if">count_if</a>
</h5></div></div></div>
<h6>
<a name="fusion.algorithm.query.functions.count_if.h0"></a>
            <span class="phrase"><a name="fusion.algorithm.query.functions.count_if.description"></a></span><a class="link" href="count_if.html#fusion.algorithm.query.functions.count_if.description">Description</a>
          </h6>
<p>
            Returns the number of elements within a sequence with a type for which
            a given unary function object evaluates to <code class="computeroutput"><span class="keyword">true</span></code>.
          </p>
<h6>
<a name="fusion.algorithm.query.functions.count_if.h1"></a>
            <span class="phrase"><a name="fusion.algorithm.query.functions.count_if.synopsis"></a></span><a class="link" href="count_if.html#fusion.algorithm.query.functions.count_if.synopsis">Synopsis</a>
          </h6>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <span class="identifier">Sequence</span><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">F</span>
    <span class="special">&gt;</span>
<span class="keyword">typename</span> <a class="link" href="../metafunctions/count_if.html" title="count_if"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">count_if</span></code></a><span class="special">&lt;</span><span class="identifier">Sequence</span><span class="special">,</span> <span class="identifier">F</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">count_if</span><span class="special">(</span>
    <span class="identifier">Sequence</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">seq</span><span class="special">,</span> <span class="identifier">F</span> <span class="identifier">f</span><span class="special">);</span>
</pre>
<div class="table">
<a name="fusion.algorithm.query.functions.count_if.t0"></a><p class="title"><b>Table 1.58. Parameters</b></p>
<div class="table-contents"><table class="table" summary="Parameters">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Parameter
                    </p>
                  </th>
<th>
                    <p>
                      Requirement
                    </p>
                  </th>
<th>
                    <p>
                      Description
                    </p>
                  </th>
</tr></thead>
<tbody>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">seq</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      A model of <a class="link" href="../../../sequence/concepts/forward_sequence.html" title="Forward Sequence">Forward
                      Sequence</a>, <code class="computeroutput"><span class="identifier">f</span><span class="special">(</span><span class="identifier">e</span><span class="special">)</span></code> is a valid expression, convertible
                      to <code class="computeroutput"><span class="keyword">bool</span></code>, for each
                      element <code class="computeroutput"><span class="identifier">e</span></code> in
                      <code class="computeroutput"><span class="identifier">seq</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      The sequence to search
                    </p>
                  </td>
</tr>
<tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">f</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      A unary function object
                    </p>
                  </td>
<td>
                    <p>
                      The search predicate
                    </p>
                  </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><h6>
<a name="fusion.algorithm.query.functions.count_if.h2"></a>
            <span class="phrase"><a name="fusion.algorithm.query.functions.count_if.expression_semantics"></a></span><a class="link" href="count_if.html#fusion.algorithm.query.functions.count_if.expression_semantics">Expression
            Semantics</a>
          </h6>
<pre class="programlisting"><a class="link" href="count_if.html" title="count_if"><code class="computeroutput"><span class="identifier">count_if</span></code></a><span class="special">(</span><span class="identifier">seq</span><span class="special">,</span> <span class="identifier">f</span><span class="special">)</span>
</pre>
<p>
            <span class="bold"><strong>Return type</strong></span>: <code class="computeroutput"><span class="keyword">int</span></code>
          </p>
<p>
            <span class="bold"><strong>Semantics</strong></span>: Returns the number of elements
            in <code class="computeroutput"><span class="identifier">seq</span></code> where <code class="computeroutput"><span class="identifier">f</span></code> evaluates to <code class="computeroutput"><span class="keyword">true</span></code>.
          </p>
<h6>
<a name="fusion.algorithm.query.functions.count_if.h3"></a>
            <span class="phrase"><a name="fusion.algorithm.query.functions.count_if.complexity"></a></span><a class="link" href="count_if.html#fusion.algorithm.query.functions.count_if.complexity">Complexity</a>
          </h6>
<p>
            Linear. At most <code class="computeroutput"><a class="link" href="../../../sequence/intrinsic/metafunctions/size.html" title="size"><code class="computeroutput"><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">size</span></code></a><span class="special">&lt;</span><span class="identifier">Sequence</span><span class="special">&gt;::</span><span class="identifier">value</span></code> comparisons.
          </p>
<h6>
<a name="fusion.algorithm.query.functions.count_if.h4"></a>
            <span class="phrase"><a name="fusion.algorithm.query.functions.count_if.header"></a></span><a class="link" href="count_if.html#fusion.algorithm.query.functions.count_if.header">Header</a>
          </h6>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">algorithm</span><span class="special">/</span><span class="identifier">query</span><span class="special">/</span><span class="identifier">count_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">count_if</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<h6>
<a name="fusion.algorithm.query.functions.count_if.h5"></a>
            <span class="phrase"><a name="fusion.algorithm.query.functions.count_if.example"></a></span><a class="link" href="count_if.html#fusion.algorithm.query.functions.count_if.example">Example</a>
          </h6>
<pre class="programlisting"><span class="keyword">const</span> <a class="link" href="../../../container/vector.html" title="vector"><code class="computeroutput"><span class="identifier">vector</span></code></a><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">vec</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><a class="link" href="count_if.html" title="count_if"><code class="computeroutput"><span class="identifier">count_if</span></code></a><span class="special">(</span><span class="identifier">vec</span><span class="special">,</span><span class="identifier">odd</span><span class="special">())</span> <span class="special">==</span> <span class="number">2</span><span class="special">);</span>
</pre>
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